Demonstration of surface-engineered oxidation-resistant Nb-Nb thermocompression bonding toward scalable superconducting quantum computing architectures

TL;DR

This paper demonstrates a surface-engineered Nb-Nb bonding method using an ultrathin gold layer to prevent oxidation, enabling scalable, low-temperature superconducting quantum device integration.

Contribution

It introduces a gold passivation technique that suppresses Nb oxidation, facilitating reliable low-temperature Nb-Nb bonding for quantum computing architectures.

Findings

Gold passivation effectively prevents Nb oxide formation.

Low-temperature bonding at 350°C achieved with high bond strength.

Enhanced bonding uniformity and superconductivity retention.

Abstract

Scalable quantum computing currently requires a large array of qubit integration, but present two-dimensional interconnects face challenges such as wiring congestion, electromagnetic interference, and limited cryogenic space. To overcome this challenge, implementing three-dimensional (3D) vertical architectures becomes crucial. Niobium (Nb), due to its excellent superconducting characteristics and strong fabrication process compatibility, stands out as a prime material choice. The main challenge in Nb-Nb bonding is the presence of an oxide layer at the interface, even after post-bonding annealing across various bonding methods. The native Nb oxide forms rapidly in air, creating a resistive barrier to supercurrent flow and introducing two-level system losses that degrade qubit coherence while increasing the overall thermal budget. These issues show the need for effective surface engineering to suppress oxidation during bonding. This study introduces an ultrathin gold (Au) capping layer as a passivation strategy to prevent oxygen incorporation at the Nb surface. This approach enables low-temperature Nb-Nb thermocompression bonding at 350 {\deg}C under a reduced bonding pressure of 0.495 MPa. Detailed microstructural and interfacial analyses confirm that Au passivation effectively suppresses oxide formation and hence enhances bonding uniformity and strength with keeping the superconductivity, establishing a robust route toward low-temperature, low-pressure Nb-Nb bonding for scalable 3D superconducting quantum computing architectures.